1. Introduction
This invention relates to a shortened process for electroless metal plating over a substrate and more particularly, to a process for electroless metal plating particularly useful in the manufacture of printed circuit boards.
2. Description Of The Prior Art
When preparing printed circuit boards having more than one circuit pattern, conductive holes, known in the art as through-holes, pass through the board to accommodate insertion and soldering of electrical component leads and to make electrical connections between two or more circuit patterns. Conductive through-holes are conventionally formed by drilling or punching holes through a copper clad, rigid board typically followed by an electroless copper plating procedure. A procedure for the formation of such printed circuit boards is disclosed in U.S. Pat. No. 4,217,182 incorporated herein by reference.
Electroless copper plating procedures used in the formation of printed circuit boards are known in the art and described in numerous publications such as by Coombs Jr. "Printed Circuit Handbook", McGraw-Hill Book Company, New York, N.Y., 1967, Chapter 5, and by Draper, "Printed Circuits and Electronic Assemblies", Robert Draper Limited, Teddington, 1969, Chapter 6, both incorporated herein by reference. The copper clad board with plated through-holes can be processed to printed circuit boards using resists and processes such as those disclosed in the aforementioned "Printed Circuit Handbook" or for example, in any of U.S. Pat. Nos. 3,469,982; 3,526,504; 3,547,730; 3,622,344; and 3,837,860; all incorporated herein by reference.
Printed circuit board failure often occurs as a consequence of poor adhesion of the metal plated onto the sidewalls of the through-holes. There are several causes of poor adhesion. For example, when a through-hole is drilled through the board, a substantial temperature increase occurs at the interface between the drill bit and the wall of the through-hole as a consequence of friction. This results in melting of the plastic substrate at the interface. The melted plastic on the surface of the hole-wall forms a smooth smear over the surface of the hole which is poorly adsorptive of plating catalyst. Another cause of poor adhesion is the presence of minute particles of the plastic formed during the drilling or punching operation. These particles may interfere with the bond between the metal plate and the wall of the through-hole. When a circuit board base material comprising a glass filled epoxy is used as the circuit board base material, the fibrils of the glass fiber often protrude into the hole and interfere with the bond between the subsequently deposited metal and the sidewall of the hole.
An advance in the art of printed circuit board manufacture is the introduction of the multilayer board where multiple circuits are stacked in layers on a circuit board and pressed into a multilayer board. Each circuit is separated from another by a layer of a dielectric material. In a manner similar to the double sided through-hole circuit board, holes are formed in the multilayer stack to form interconnections between the same.
Adhesion of copper to sidewalls of through-holes is more critical in the manufacture of multilayer boards than in the manufacture of two-sided through-hole boards. However, because there are a stack of multiple circuits, through-hole plating is more difficult than through-hole plating of double sided boards and satisfactory and consistent adhesion of the copper to the sidewall is more difficult to achieve.
The basic steps required to cause an electroless metal to deposit on a dielectric surface comprise immersion of the surface into a catalyst composition such as a tin-palladium colloid as disclosed in U.S. Pat. No. 3,011,920. Following catalysis, and a step of acceleration as described in the aforesaid patent, an electroless metal will deposit onto the catalyzed surface. The result is a coherent film of electroless metal over the dielectric surface which for many applications is sufficiently adherent to the dielectric surface. In the manufacture of printed circuit boards, especially in the plating of through-holes, a process comprising only the steps of catalysis and metal plating will not provide a metal deposit with adequate adhesion between the dielectric surface and the deposit. Accordingly, metallization of through-holes in printed circuit board manufacture typically involves a plating line having several steps preceding catalysis and metallization. A plating line in commercial use in the manufacture of plated through-hole printed circuit boards is disclosed in U.S. Pat. No. 4,515,829 incorporated herein by reference. Example 2 of this patent discloses the conventional process which involves ten steps following the step of drilling through-holes though the first step of sulfuric acid treatment is optional. The process provides a circuit board having excellent adhesion between the dielectric surface and the copper.
One object of this invention is to provide a novel plating line for the metallization of dielectric surfaces which is useful in the manufacture of plated through-hole printed circuit boards, utilizes fewer processing steps and is therefore more economical than existing plating lines. Another object of this invention is to provide novel compositions for use in the plating process of the invention.